1. Field of the Invention
The present invention relates to a control system for a hybrid vehicle having an engine for generating propulsive forces for the hybrid vehicle and an electric motor for generating assistive forces in addition to the propulsive forces generated by the engine and absorbing the propulsive forces generated by the engine.
2. Description of the Related Art
There has in recent years been developed a hybrid vehicle which has an engine, like an internal combustion engine for ordinary automobiles, as a power source for generating propulsive forces (running drive forces) for the hybrid vehicle and also an electric motor connected to the output shaft of the engine for generating assistive drive forces in addition to the propulsive forces generated by the engine, i.e., applying assistive drive forces to the output shaft of the engine, and for being operated as an electric generator by the engine (regenerative mode) to convert and absorb the propulsive forces generated by the engine as electric energy. For details, reference should be made to Japanese patent publication No. 6-1925, for example. To generate the assistive drive forces, the electric motor is energized by electric energy from a power supply such as a storage battery. When it is operated as the electric generator, the generated electric energy is stored in the storage battery.
Vehicle engines should preferably be operated at a constant rotational speed when idling in order to prevent itself from excessively vibrating, minimize fuel consumption, and avoid excessive exhaust gas emission. For controlling the engine to idle at a constant rotational speed, it has heretofore been the general practice to adjust the rate of an air/fuel mixture introduced into the engine. Specifically, if the rotational speed of the engine rises due to a decrease in the engine load while the engine is idling, then the rate of the air/fuel mixture introduced into the engine is reduced to lower the rotational speed of the engine. Conversely, if the rotational speed of the engine falls due to an increase in the engine load while the engine is idling, then the rate of the air/fuel mixture introduced into the engine is increased to increase the rotational speed of the engine. In this manner, the rotational speed of the engine is maintained at the constant level.
The above engine rotation control, however, generally suffers a response delay until the rotational speed of the engine actually reaches a desired value after the rate of the air/fuel mixture introduced into the engine is adjusted. Therefore, when the rotational speed of the engine fluctuates at short time intervals, such speed fluctuations cannot sufficiently be eliminated by the above engine rotation control.
On the above hybrid vehicle, when the assistive drive forces are generated by the electric motor, the load on the engine is reduced, increasing the rotational speed thereof. When the electric motor is operated as the electric generator to generate electric energy, the load on the engine is increased, lowering the rotational speed thereof.
Therefore, it is possible to control the electric motor to generate the assistive drive forces in order to cancel variations in the rotational speed of the engine, or to generate electric energy in order to suppress fluctuations in the rotational speed of the engine.
For suppressing a fluctuation in the rotational speed of the engine, such a rotational speed fluctuation is detected while the engine is idling, an assistive drive force or electric energy to be generated by the electric motor for suppressing the rotational speed fluctuation is determined using a map or a data table from the detected rotational speed fluctuation, and the electric motor is controlled on the basis of the determined assistive drive force or electric energy.
According to the above motor control process, however, the operating state of the electric motor varies immediately in response to even a slight variation in the rotational speed of the engine. The electric motor tends to apply excessive forces to the rotational shaft of the engine in an attempt to cancel out the variation in the rotational speed of the engine, causing another slight variation in the rotational speed of the engine.